427e574bd1
since the same function can be achieved by mount:
mount(NULL, "/mnt/unionfs", "unionfs", 0,
"fspath1=/mnt/path1,prefix1=prefix1,"
"fspath2=/mnt/path2,prefix2=prefix2");
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
94 lines
4.3 KiB
Plaintext
94 lines
4.3 KiB
Plaintext
fs/unionfs/README.txt
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=====================
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Overview
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--------
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This directory contains the NuttX Union File System. The Union file
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system is provides a mechanism to overlay two different, mounted file
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systems so that they appear as one. In general this works like this:
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1) Mount file system 1 at some location, say /mnt/file1
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2) Mount file system 2 at some location, say /mnt/file2
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3) Call mount() to combine and overly /mnt/file1 and mnt/file2
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as a new mount point, say /mnt/unionfs.
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/mnt/file1 and /mnt/file2 will disappear and be replaced by the single
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mountpoint /mnut/unionfs. The previous contents under /mnt/file1 and
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/mnt/file2 will appear merged under /mnt/unionfs. Files at the same
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relative path in file system1 will take presence. If another file of the
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same name and same relative location exists in file system 2, it will
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not be visible because it will be occluded by the file in file system1.
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See include/nutts/unionfs.h for additional information.
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The Union File System is enabled by selecting the CONFIG_FS_UNIONFS option
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in the NuttX configuration file.
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Disclaimer: This Union File System was certainly inspired by UnionFS
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(http://en.wikipedia.org/wiki/UnionFS) and the similarity in naming is
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unavoidable. However, other than that, the NuttX Union File System
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has no relationship with the UnioinFS project in specification, usage,
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design, or implementation.
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Uses of the Union File System
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------------------------------
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The original motivation for this file was for the use of the built-in
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function file system (BINFS) with a web server. In that case, the built
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in functions provide CGI programs. But the BINFS file system cannot hold
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content. Fixed content would need to be retained in a more standard file
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system such as ROMFS. With this Union File System, you can overly the
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BINFS mountpoint on the ROMFS mountpoint, providing a single directory
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that appears to contain the executables from the BINFS file system along
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with the web content from the ROMFS file system.
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Another possible use for the Union File System could be to augment or
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replace files in a FLASH file system. For example, suppose that you have
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a product that ships with content in a ROMFS file system provided by the
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on-board FLASH. Later, you overlay that ROMFS file system with additional
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files from an SD card by using the Union File System to overlay, and
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perhaps replace, the ROMFS files.
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Another use case might be to overlay a read-only file system like ROMFS
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with a writable file system (like a RAM disk). This should then give
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to a readable/write-able file system with some fixed content.
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Prefixes
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--------
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And optional prefix may be provided with each of the file systems
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combined in by the Union File System. For example, suppose that:
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o File system 1 is a ROMFS file system with prefix == NULL,
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o File system 2 is a BINFS file system with prefix == "cgin-bin", and
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o The union file system is mounted at /mnt/www.
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Then the content in the in the ROMFS file system would appear at
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/mnt/www and the content of the BINFS file system would appear at
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/mnt/www/cgi-gin.
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Example Configurations
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----------------------
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o boards/sim/sim/sim/unionfs - This is a simulator configuration that
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uses the Union File System test at apps/examples/unionfs. That test
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overlays two small ROMFS file systems with many conflicts in
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directories and file names. This is a good platform for testing the
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Union file System and apps/examples/unionfs is a good example of how to
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configure the Union File System.
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o boards/arm/lpc17xx_40xx/lincoln60/thttpd-binfs - This is an example
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using the THTTPD web server. It server up content from a Union File
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System with fixed content provided by a ROMFS file system and CGI
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content provided by a BINFS file system.
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You can see how the Union File System content directory is configured
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by logic in apps/example/thttpd/.
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o boards/arm/lpc17xx_40xx/olimex-lpc1766stk/thttpd-binfs - This is
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essentially the same as the lincoln60 configuration. It does not work,
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however, because the LPC1766 has insufficient RAM to support the THTTPD
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application in this configuration.
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See the README.txt file in each of these board directories for additional
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information about these configurations.
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